Analog and digital detail enhancement, known as "unsharp masking", is well known and widely used in the graphic arts, as well as in other fields. It is used to produce sharp printed reproductions from original images by enhancing the color density differences at edges between areas of different color and is discussed in the following publications:
Jackson, Lonnie L., "Unsharp Masking: Photographic/Electronic," Gatfworld, May-June 1989, Vol. 1, pp. 13-22;
Molla, R. K., Electronic Color Separation, R. K. Printing & Publishing Co., Montgomery, W. Virginia, 1988, pp. 230-232; and
U.S. Pat. No. 4,335,407 to Atoji et al.
Two methods are used for detail enhancement, the utilization of a single color channel to produce a single detail enhancement signal for each of three or four color channels or for each of the three or four printers and the utilization of each color channel to produce a separate detail enhancement signal per channel. These two methods can be implemented in both analog and digital systems, such as color separation scanners.
The first method adds to the enhanced edges a generally gray line, making the edge of a color area appear to be more gray. This effect, often referred to as "contours", is undesirable for most graphic arts applications.
The second method adds a color enhancing line to the edge of a color area, thereby generally enhancing the color separation of the area at the edge to more sharply define the color edge. This effect is desirable and produces printed images of high quality.
The two methods have their advantages and disadvantages, particularly in regard to color noise. When scanning a color area, many pixels are produced to represent the area and the average color of the pixels in the area is the color of the area. However, due to noise that is inherent in the detection process, an individual pixel will typically have a color which is slightly different from the average color of the area. This effect is known as color noise and is easily detected by the human eye, especially when the average color is gray.
The second method emphasizes this effect by adding a color component to each pixel. On the other hand, the first method adds a gray component to each pixel, thereby reducing the color separation in the pixel and consequently, reducing the color noise.
At the edges between gray areas, where typically the Red, Green and Blue components of each gray color are essentially of the same color, both methods provide a sharpening line of the same average color and are, therefore, equivalent. The second method, however, will increase any color noise in the gray areas and the first method will reduce it.
From the above discussion, it is clear that neither method is superior than the other in all situations. The desirability of each method depends on the colors in the image.